NMR investigation on the honeycomb iridate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ag</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi>LiIr</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>
Jiaming Wang, Weishi Yuan, Takashi Imai, Philip M. Singer, Faranak Bahrami, Fazel Tafti
Abstract
${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$ is a Kitaev spin-liquid candidate material synthesized from $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ through a topotactic reaction. We investigate the structural and magnetic properties of two samples of ${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$ based on the $^{7}\mathrm{Li}$ nuclear magnetic resonance line shape, Knight shift, and spin-lattice relaxation rate $1/{T}_{1}$. The first sample A shows signatures of magnetically ordered spins, and exhibits one sharp $^{7}\mathrm{Li}$ peak with the full width at half maximum increasing significantly below 14 K. $1/{T}_{1}^{\text{stretch}}$ of this sample displays a broad local maximum at 40 K, followed by a very sharp peak at ${T}_{N}=9\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ due to a critical slowing down of Ir spin fluctuations, a typical signature of magnetic long-range order. In order to shed light on the position-by-position variation of $1/{T}_{1}$ throughout the sample, we use a numerical inverse Laplace transform ${T}_{1}$ analysis based on Tikhonov regularization to deduce the density distribution function $P(1/{T}_{1})$. We demonstrate that $\ensuremath{\sim}60%$ of Ir spins are statically ordered at the NMR measurement timescale but the rest of the sample volume remains paramagnetic even at 4.2 K, presumably because of structural disorder induced primarily by stacking faults. In order to further investigate the influence of structural disorder, we compare these NMR results with those of a second sample B, which has been shown by transmission electron microscopy to have domains with unwanted Ag inclusion at the Li and Ir sites within the Ir honeycomb planes. Sample B displays an additional NMR peak with a relative intensity of $\ensuremath{\sim}17%$. The small Knight shift and $1/{T}_{1}$ of these defect-induced $^{7}\mathrm{Li}$ sites and the enhancement of bulk susceptibility at low temperatures suggest that these defects generate domains of only weakly magnetic Ir spins accompanied by free spins, leading to a lack of clear signatures of long-range order. The apparent lack of long-range order could be easily misinterpreted as evidence for the realization of a spin-liquid ground state in highly disordered Kitaev lattices.